Various coatings have been applied to paperboard substrates to provide composite materials that may be used for various purposes. In particular, barrier coatings have been applied to paperboard substrates used to make food containers in order to reduce the transport of moisture and oxygen into the food that may spoil or contaminate the food or otherwise disaffect its flavor or shelf life.
Accordingly, for food container applications, paper and paperboard substrates are conventionally coated with barrier coatings selected from various polymers such as polyethylene which provides a moisture barrier and ethylene-vinyl alcohol copolymer (EVOH) to address flavor or property-affecting atmospheric changes in the containers. Such coatings can reduce transmission of moisture and oxygen through the packaging material and provide for longer shelf life for the food packaged in the container. However, some oxygen sensitive foods, such as orange juice, tomato products and fresh meat still have only a relatively short life of a few weeks despite the barrier coatings. The short shelf-life is due, at least in part, to oxidation caused by the oxygen left in the headspace during the package filling step and/or from leakage through the container or through the barrier coatings on the container.
Methods have been devised for reducing the amount of oxygen which is transported through the container. For example, U.S. Pat. No. 5,153,061 to Cavagna et al. describes a barrier coating that may be applied to the inner or outer surface of a paperboard substrate and which absorbs a contaminate emitted by the substrate or provides a tortuous path so that the migration of the contaminate through the barrier layer is reduced. The material used for absorbing contaminants is activated carbon and the ingredient for providing a tortuous path is a delaminated clay pigment. In order to provide a tortuous path layer, the pigment and activated carbon are dispersed in a water soluble binder such as polyvinyl alcohol and applied to the substrate, and a polyethylene terephthalate layer is then coated over the tortuous path layer. While the coatings of Cavagna et al. are said to be effective in reducing the transport of undesirable substances into the food, the process for applying the coating requires multiple steps that significantly increase the cost of producing such coated paperboard products. The clay and carbon coatings of Cavagna et al. do not have oxygen scavenging properties.
U.S. Pat. Nos. 5,202,052 and 5,364,555 to Zenner et al. describe polymeric material carriers containing oxygen scavenging material. The polymeric carriers for the oxygen scavenging material include polyolefin, PVC, polyurethanes, EVA and PET. In the '555 patent, the oxygen scavenging material is a transition metal complex or chelate of a salicylic acid or salicylate salt. In the '052 patent, the oxygen scavenging material is said to be a metal complex or chelate of an organic polycarboxylic acid, preferably an amino polycarboxylic acid. The transition metals include iron, copper, cobalt, or nickel. The material of Zenner et al. is disclosed for use as crown or closure liners for packaging materials, as a gasket or liner applied to an aluminum or plastic crown for plastic or glass bottles, and as a plastic from which plastic bottles may be made. According to Zenner et al., the oxygen scavenging material is activated by contact with water or water vapor. However, a paperboard coated with these materials may be activated prematurely from atmospheric oxygen surrounding the container and thus lose its effectiveness as an oxygen scavenger over time.
U.S. Pat. No. 5,529,833 to Speer et al. describes the use a composition comprising an ethylenically unsaturated hydrocarbon oxygen scavenger which is incorporated into a layer such as film layer and used for making packaging for oxygen-sensitive products. The oxygen scavenger is catalyzed by a transition metal catalyst selected from salts derived from the first, second or third series of transition metals of the Periodic table and a counterion selected from a chloride, acetate, stearate, palmitate, 2-ethylhexanoate, neodecanoate or naphthenate. Preferred metal salts are selected from cobalt (II) 2-ethylhexanoate and cobalt (II) neodecanoate. The components of the composition are mixed by melt-blending at a temperature in the range of 50.degree. to about 300.degree. C. Because water deactivates the oxygen-scavenger composition, the composition can only be used for packaging for dry materials. Furthermore, the oxygen scavenger composition is always active once made due to the presence of the catalyst. Accordingly, the composition loses a considerable amount of its scavenging capacity over time and thus cannot be stored in final form for an extended period of time before use.
Other known materials for reducing the oxygen transport to a food substance include antioxidants such as butylated hydroxyanisole, di-tertiary-butyl-paracresol, propyl gallate, phenylenethiourea and aldolalpha-naphthylamine and granular metals and metal salts contained in water and oxygen permeable packages.
Despite the use of many types of coatings, there remains a need for selection and treatment of a substrate, such as paperboard with a coating which can provide an oxygen scavenging in order to reduce the head space oxygen in the container and thereby reduce the oxidation of the constituents of the food contained therein. Such coatings should also lend themselves to lower cost production techniques.
Accordingly, it is an object of the invention to provide an improved oxygen scavenging blend for use in coating substrates for food packaging applications.
Another object of the invention is to reduce the manufacturing costs of paperboard products containing oxygen barriers.
A further object of the invention is to provide a barrier layer for a paperboard product that can be extruded in a single or multiple operational step.
Still another object of the invention is to produce a container from such a paperboard product containing an improved oxygen scavenging blend.
A unique advantage of this invention is the produced by-product, carbon dioxide. This gas slows down oxygen leakage from outside the container, displaces dissolved oxygen and protects the product from oxidation.
Yet another object of the invention is to provide an extrusion method for applying an oxygen scavenger-containing barrier layer to a paperboard product for use in making food containers.
A additional object of the invention is to provide an oxygen scavenging blend which could either be coated on to a substrate such as paper, paperboard or a non-woven by a process such as mono- or co-extrusion coating or lamination; or used directly after conversion into a film or bag or box or other container by processes known to those skilled in the art, such as blown or cast film or sheet extrusion, blow molding, injection molding, metal or other insert molding, thermoforming, vacuum thermoforming, etc. for the packaging of food and other products.